1. Field of the Invention
The present invention relates to a webbing winding device which winds up and accommodates a webbing belt which is for restraining the body of a vehicle occupant.
2. Description of the Related Art
A seatbelt apparatus, which restrains the body of an occupant seated on a seat of a vehicle with a long belt-form webbing belt, is equipped with a webbing winding device which is fixed to the vehicle body sideward of the seat. The webbing winding device is provided with, for example, a spool whose axial direction is arranged substantially along a front-rear direction of the vehicle. A length direction base end side of the webbing belt is anchored at this spool. The spool accommodates the webbing belt in a state in which the webbing belt is wound up in a layered form at an outer peripheral portion of the spool.
This kind of webbing winding device is also usually provided with a locking mechanism. Such locking mechanisms have various structures but, in functional terms, the locking mechanism locks the spool at a time of sharp deceleration of the vehicle, and restricts rotation of the spool when the webbing belt is being unwound. In a state of sharp deceleration of a vehicle, an occupant of the vehicle tends to move toward a forward side of the vehicle because of inertia. At this time, the webbing belt which is applied to the body of the occupant is pulled on by the body of the occupant.
Now, as mentioned above, when the spool is locked by the locking mechanism, unwinding of the webbing belt from the spool is restricted. Consequently, the movement of the body of the occupant toward the forward side of the vehicle due to inertia in the state of sharp deceleration of the vehicle can be effectively restricted by the webbing belt.
Further, there is a structure which is provided with a torsion shaft for energy absorption at an inner side of the spool. This torsion shaft is provided coaxially at the inner side of the spool to be rotatable with respect to the spool. One end portion of the torsion shaft is integrally joined to the spool.
Another end portion of the torsion shaft is joined to the aforementioned locking mechanism. When the locking mechanism operates, the other end portion of the torsion shaft is locked by the locking mechanism. When a rotation force acts on the spool, the torque is, naturally, also applied to the one end of the torsion shaft. However, in a state in which the locking mechanism has operated, because rotation of the other end of the torsion shaft is restricted, rotation of the torsion shaft as a whole is limited as a result, and rotation of the spool is indirectly restricted.
However, in the state in which rotation of the other end of such a torsion shaft is restricted by the locking mechanism, if a rotation force exceeding a mechanical strength of the torsion shaft acts on a portion of joining between the spool and the torsion shaft, a twisting deformation occurs in the torsion shaft, and rotation of the spool by an amount corresponding to this twisting deformation is enabled.
Therefore, at a time when the body of an occupant pulls on the webbing belt with an excessive force, the webbing belt is unwound by a certain amount, and a restraining force that the webbing belt applies to the body of the occupant is temporarily slightly reduced. At the same time, energy absorption is realized by the above-mentioned twisting deformation.
In a webbing winding device which is disclosed in U.S. Pat. No. 5,799,893 , a torsion shaft is joined to a spool at an axial direction central portion of the spool, and locking mechanisms are provided at both ends of the spool for respectively restricting rotation of the torsion shaft.
In the webbing winding device disclosed in U.S. Pat. No. 5,799,893 , one or both of the two locking mechanisms is operated, depending on the physical build of an occupant or the like. For the case in which both the locking mechanisms are operated, a rotation force required for causing twisting deformation of the torsion shaft must exceed a mechanical strength of the whole of the torsion shaft.
On the other hand, when only one of the locking mechanisms is operated, twisting deformation occurs in the torsion shaft if the mechanical strength of the torsion shaft at the side of the one locking mechanism, relative to the portion of joining of the torsion shaft with the spool, is exceeded.
In other words, in the webbing winding device disclosed in U.S. Patent No. 5,799,893, it is possible to switch the magnitude of a rotation force that is required for causing twisting deformation of the torsion shaft.
Now, in the webbing winding device disclosed in U.S. Pat. No. 5,799,893 , in order to operate both of the locking mechanisms, it is necessary to cause both the one locking mechanism and the other locking mechanism to operate. Obviously, it is possible to independently constitute the two locking mechanisms each with the same structure. However, with such a constitution, a number of components is large and the webbing winding device as a whole becomes larger.
Therefore, a structure which joins to the one locking mechanism and operates the other locking mechanism is desirable. However, in order to join any member of the other locking mechanism to any member of the one locking mechanism, an interlinking member which crosses between the two locking mechanisms is required.
However, because the rotating spool is disposed between the two locking mechanisms, the interlinking member is provided so as to go round the outer side of the spool. An interlinking member which goes round the outer side of the spool in such a manner cannot be formed in a structure which rotates together with the spool. Accordingly, in a case in which a member at the one locking mechanism for moving the interlinking member is a pawl which restricts rotation of the spool by meshing with a ratchet tooth, it is not possible to form a structure in which the pawl is turned together with the spool.
Therefore, the ratchet tooth that the pawl meshes with must be formed in a structure which is provided integrally with the torsion shaft, and it is extremely difficult to apply the interlinking to a structure in which a pawl rotates together with the spool and restricts rotation of the spool by meshing with a ratchet tooth formed in a frame.